The present invention relates to an electric resistance heating element made of a carbon material and, more specifically, to an improved electric resistance heating element formed by providing around a tube a layer essentially comprising carbon fiber, as well as a heating furnace using this heating element.
As high-temperature heating furnaces employed for manufacture of various industrial materials such as carbon fiber, graphite fiber and other carbon materials and also ceramics or the like, there have been known a great variety of industrial furnaces such as electric resistance heating furnaces, induction heating furnaces, arc heating furnaces, plasma heating furnaces, etc.
Among these high-temperature heating furnaces, the Tamman heating furnace (hereinafter referred to as "Tamman furnace") wherein a cylindrical carbon material is employed and electrodes are provided at both ends thereof across which electric current is supplied for heating, is particularly widely employed as a heating furnace for the manufacture of the above-mentioned industrial materials, since the heating means thereof is relatively simple.
The above-mentioned Tamman furnace has, such a structure in which a cylindrical heating tube made of a carbon material is surrounded by a thermal insulating material, and the inside of the heating tube thereof is a heat treatment chamber, in which an object to be thermally treated is disposed or passed, and a current is supplied between the electrodes provided at both ends of the heating tube in order to generate Joule heat for heating the object the heat treatment chamber. Generally, the temperature of the furnace is extremely high and the inside of the heat treatment chamber is maintained under an atmosphere of an inert gas, such as nitrogen, argon, helium or the like, or a pressure having been reduced or a vacuum.
Carbon or graphite materials are employed as the heating tube of the Tamman furnace, because of its thermal stability, i.e., these materials never fuse or pyrolytically decompose even in a high-temperature region of 2000.degree. to 3000.degree. C., and function satisfactorily as an electric heating element.
However, the heating tube itself is gradually exhausted in case of heating over a long period of time under a temperature of 2000.degree. to 3000.degree. C. or higher than the same in a Tamman furnace having the above-mentioned structure. Moreover, as the heating tube is exhausted, the electric resistance of the heating tube fluctuates and consequently, the temperature profile inside the furnace changes, and this causes a problem. In other words, there is such a problem that since the change or fluctuation in the temperature profile or distribution inside the furnace may cause the change in the quality and performance of the products treated by the furnace, it becomes impossible to continue using the furnace as it is any more.
When a detectable change of the temperature inside the furnace is perceived, due to the exhaustion of the heating tube, the element should necessarily be replaced by a new one, and it is an essential matter for an industrial furnace, to minimize the period of replacement of heating tubes, since not only such replacement of heating tubes is highly costly but also there is a need for much labor and time to maintain safety in the replacement operation and cool, disassemble and assemble the furnace as well as heat up the furnace after assembling and moreover, the energy loss is not small.
The inventors have attained the present invention as the result of examinations of the factors in generation of the above-mentioned shortcomings of the high-temperature heating furnaces. In other words, paying attention to the fact that there is a relation between the life of the heating tube and the material surrounding the periphery thereof, the inventors examined various materials, and as a result, the inventors have attained the present invention through a confirmation that an excellent result can be obtained when carbon fiber is employed as a principal material.
It is, therefore, an object of the present invention to provide a resistance heating element for an industrial furnace capable of prolonging the life thereof. Another object of the present invention is to provide an improved heating element with high performance, of low manufacturing cost as well as easy to manufacture for such an industrial heating furnace.